Acacia mellifera (M. Vahl) Benth. was renamed Senegalia mellifera (Vahl) Seigler & Ebinger in 2010 (USDA, 2014). However, since most papers refer to Acacia mellifera, it was decided to use this taxon in the datasheet until Senegalia mellifera gains more widespread recognition.

The black thorn (Acacia mellifera (M. Vahl) Benth.) is an African shrub or small tree growing to a height of 9 m. It has an extensive root system that explores large volumes of soils, allowing survival in dry areas. It has a tangled, balled-shaped or flat-topped canopy that may reach down to ground level. The branches bear pairs of black hooked thorns every 5 to 15 mm. The leaves are bipinnate with only 1-2 (-4) pairs of pinnae each bearing 1-2 (-3) pairs of ovate or obovate leaflets. Leaflets are 3.5-15 mm long x 2-12 mm broad. Initially green, black thorn leaves become glaucous with maturity. The flowers are fragrant, sweetly scented, 3-5 cm long and creamy white in colour, borne in dense hanging spikes. The fruits are straw coloured flat pods, 3-8 cm long x 1.5-2.5 cm wide, that contain three seeds. The tree lives less than 10 years (FAO, 2014; Nonyane, 2013; Orwa et al., 2009; Hines et al., 1993).

The black thorn is a multipurpose tree. The leaves, pods and young shoots are nutritious and make fodder for livestock and wild animals. They are browsed by camels, goats and wild animals such as black rhinos, kudus, elands and giraffes. Goats are particularly fond of the leaves, which may constitute a considerable part of their diet. Fallen browse, dry leaves and pods are eaten on the ground by cattle and sheep who, unlike camels and goats, are less likely to browse them. The flowers are attractive to bees, which produce a high quality honey, hence the name mellifera. The timber is hard, almost black when polished, resistant to termites and used for construction and fencing. Black thorn trees are used for live fencing and hedging (FAO, 2014; Nonyane, 2013; Orwa et al., 2009; Hines et al., 1993). The bark is used in ethnomedicine to treat stomach problems, sterility, pneumonia, malaria and syphilis (Rulangaranga, 1989 cited by Hines et al., 1993).

Distribution

The black thorn is a drought-resistant species widely spread in arid and semi-arid areas of Africa and the Arabian Peninsula. In Kenya, it can be found from 1000 to 1400 (-1500) m above sea level. It grows better on sandy, clayey or stony-rocky soils but it is tolerant of a wide range of soils, including black cotton soils (vertisols). The black thorn is found in regions with 400-800 mm annual rainfall but it can grow in areas with a minimum of 100 mm rainfall, and along seasonal watercourses or drainage networks. Black thorn trees grow in groups of pure, dense, impenetrable and even-aged thickets (FAO, 2014; Nonyane, 2013; Orwa et al., 2009).

Forage management

The black thorn can be vegetatively propagated or sown. It is a slow-growing tree somewhat sensitive to cold and frost in the early stages of development. The young trees are very palatable to livestock and should be protected during the first two years. However, bush encroachment can occur: when left unattended, the trees form dense, impenetrable thickets, sometimes hundreds of metres across, slowly taking over grazing land (Nonyane, 2013; Orwa et al., 2009). This phenomenon can be controlled with fire (FAO, 2014; Orwa et al., 2009).

Environmental impact

Shelter and shade provider

Bush encroachment

Because they can form dense impenetrable thickets, black thorns may outcompete grasses in dry areas and become the only remaining species, causing a considerable reduction in seasonal grass yields (32-40% in Zimbabwe and 41% in the Eastern Cape) (Kraaij et al., 2006).

Nutritional aspects

Nutritional attributes

Black thorn foliage has a high protein content (15 to 27% DM) and variable amounts of fibre (ADF 17-34% DM). As in other browse species, the composition is extremely variable due to the different proportion of leaves, petioles and stems collected by researchers or browsed by animals.

Potential constraints

Like other Acacia species, black thorn foliage contains tannins, though the reported tannin content is extremely variable. Condensed tannins have been found to be present either in low amounts (0.2% DM, Osuga et al., 2007), or in much larger quantities: in Sudan, from 1.4% DM in the early dry season (late October) to 5.7% DM in the late dry season (early June) (Fadel Elseed et al., 2002).

Ruminants

The black thorn is a good browse species for all classes of domestic and wild ruminants in arid and semi-arid areas. Particularly, it provides valuable protein supplementation to livestock. For instance, it plays a major role in the protein supply of goats in Kenya, to the extent that overuse of black thorn for firewood is of concern for goat production (Stuth et al., 1990). It may require P supplementation (Fadel Elseed et al., 2002).

Digestibility

In a comparison of 15 Kenyan indigenous browse species, Acacia mellifera was in the middle of the range for in vitro DM degradability (56%) and in vitro (gas production) OM digestibility (54%), with lower values than those of Maerua angolensis and Ziziphus mucronata (Ondiek et al., 2010). In experiments aimed at alleviating the effect of acacia tannins on protein digestibility, the addition of PEG (polyethylene glycol) to black thorn foliage did not increase gas production, probably because the black thorn does not contain large amounts of tannins (Osuga et al., 2007; Wambui et al., 2012). The addition of brewer’s yeast increased gas production in a browse mixture containing Berchemia discolor and black thorn in a 1:1 ratio (Wambui et al., 2012).

Other species

Bees

During the flowering period, bees forage in the late morning to mid-afternoon when the weather is hot and dry, resulting in a tasty, slow granulating honey (Orwa et al., 2009).